1. Field of the Invention
This invention relates to a method for displaying an energy subtraction image obtained from subtraction processing, in particular, digital subtraction processing, which is carried out on radiation images, and an apparatus for carrying out the method.
2. Description of the Prior Art
Techniques for carrying out digital subtraction processing on radiation images have heretofore been known. When digital subtraction processing is to be carried out, two radiation images recorded under different conditions are photoelectrically read out, and digital image signals which represent the radiation images are obtained. The image signal components of the digital image signals which represent corresponding picture elements in the radiation images are then subtracted from each other, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method, two digital image signals are subtracted from each other in order to obtain a difference signal, and the radiation image of a specific structure can be reproduced from the difference signal.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order to extract the image of a specific structure of an object from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media. In the latter method, such characteristics are utilized that a specific structure of an object exhibits specific radiation energy absorbing characteristics. Specifically, an object is exposed to several kinds of radiation with different energy distributions. In this manner, two radiation images, in which different images of a specific structure are embedded, are obtained. Thereafter, the image signals representing the two radiation images are weighted appropriately, and the weighted image signals are subjected to a subtraction process. The image of the specific structure is thereby extracted.
Subtraction processing is extremely effective, particularly for medical diagnosis utilizing image processing of X-ray images, and electronics research has continued to develop improved subtraction processing methods.
By storing radiation images of an object, which is constituted of bones and soft tissues, on two stimulable phosphor sheets and carrying out energy subtraction processing on image signals representing the radiation images, a subtraction image can be obtained in which the patterns of the bones have been erased and only the patterns of the soft tissues have been extracted. (Such a subtraction image will hereinbelow be referred to as the "soft tissue image".) Alternatively, a subtraction image can be obtained in which the patterns of the soft tissues have been erased and only the patterns of the bones have been extracted. (Such a subtraction image will hereinbelow be referred to as the "bone image".) However, in such cases, the soft tissue image or the bone image can only be observed independently.
With an image (e.g. an image of the chest of a human body) including both the bone patterns and the soft tissue patterns, both patterns can be observed simultaneously. For medical diagnosis, such an image is convenient from the point of view in that, for example, the relationship between the positions of the bone patterns and the soft tissue patterns can be found easily.
Also, a need exists for an image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. Such an image can be obtained by carrying out energy subtraction processing, generating an image signal representing only a soft tissue image or a bone image, and thereafter carrying out an image processing operation in a manner suitable for the image signal. For example, it is also desired that, only the patterns of edges or ridges of bones be emphasized in a bone image, or that only the patterns of interstitial tissues be emphasized in a soft tissue image.
Therefore, it will be very advantageous for diagnosis if image processing operations can be carried out independently on the image signal representing a bone image and the image signal representing a soft tissue image, and an image can be displayed in which the bone image and the soft tissue image are superposed one upon the other. For example, it will be very convenient for diagnosis if an image can be displayed in which only the patterns of edges or ridges of bones are emphasized, while patterns of blood vessels, or the like, are not particularly emphasized, or if an image can be displayed in which the patterns of bones are unsharp and only the patterns of interstitial tissues, such as the lung fields, are emphasized.